new feature and discussion: filtering on pids

[Martin Hunt <hunt at redhat dot com>]

I've checked in some new options that allow systemtap to focus on a
specific pid. Dtrace does the same with the "-c" option and $target
macro. I'd like to propose some further changes, but first here is what
I checked in:

Say you have a very simple script:

probe kernel.function("sys_open") {
        log (execname()." opens ".user_string($filename))
}

You want to use this to see what files xemacs is opening. So you start
your script, run xemacs, stop xemacs, then stop the script. And
somewhere in the middle is the output you want.  With the "-c" option
you can do that much simpler.

> stap -c "xemacs foo.c" opens.stp

And now the script starts and ends with the command you started, "xemacs
foo.c".  However, you still have some opens in your output that were
from other processes on your system.  To filter them out, modify
opens.stp like this:

probe kernel.function("sys_open") {
        if (target() == pid())
                log (execname()." opens ".user_string($filename))
}

Now you will get every sys_open() call made by the xemacs process you
started and nothing else.

There is a new related stap option, "-x pid".  It causes target() to
return whichever pid you pass in.

> stap -x 1234 opens_pid.stp
would return every file process 1234 opens. (Note that if process 1234
exits, systemtap currently won't exit.  You will have to ^C it like
normal.)

While doing this, I got to thinking that it would be nice if I didn't
have to modify my scripts to check the current pid against target()
everywhere.  Why not make this automatic? It would be easy enough to
have the translator put some code at the very beginning of each probe to
check if a target pid was set and if so don't execute the probe unless
it matched the current pid. In fact I tried it and it works very nicely.
ANY script can be made to apply only to a specific process simply by
using the "-c" or "-x" options. No changes to the script are required.

Then I thought about tracking child processes and client-server
interaction and I realized that this wouldn't cover those.  Do we need
to extend this feature to include filtering on a list of processes?
Should automatic filtering on pid be enabled by a separate flag? Or
should there be alternate options (like "-C" and "-X") that work like
"-c" and "-x" but enable the automatic filtering?

Martin